The present invention relates to a flow-through device for measuring the platelet function of primary hemostasis, the aggregation and/or the coagulation and/or the viscosity of the blood, the blood from a blood reservoir being aspirated with the help of a piston, which can be moved in a cylinder, through an aperture and the pressure in the space between the piston and the aspirated blood being measured. At the same time, the piston is moved by a driving mechanism, for example, in such a manner, that a nominal pressure is maintained in the space. The movement of the piston serves then as a measure of the amount of blood flowing through the aperture. Such a device is disclosed in the EP 0 223 044 B1.